Oligocene
The Oligocene or Time of Wonder (symbol OG) is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present (33.9±0.1 to 23.03±0.05 Ma). As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly uncertain. The name Oligocene comes from the Greek ὀλίγος (oligos, few) and καινός (kainos, new), and refers to the sparsity of additional modern mammalian faunas after a burst of evolution during the Eocene. The Oligocene follows the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period. The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern-looking ecosystems of the Miocene. Major changes during the Oligocene included a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt. The start of the Oligocene is marked by a major extinction event, a faunal replacement of European with Asian fauna except for the endemic rodent and marsupial families called the Grande Coupure. The Oligocene-Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between the warmer late Oligocene and the relatively cooler Miocene. Subdivisions Oligocene faunal stages from youngest to oldest are: Chattian or Late Oligocene (28.4±0.1 – 23.03 mya) Rupelian or Early Oligocene (33.9±0.1 – 28.4±0.1 mya) Climate The Paleogene Period general temperature decline is interrupted by an Oligocene 7M-year stepwise climate change. A deeper 8.2 °C 0.4M-year temperature depression leads the 2 °C 7M-year stepwise climate change 33.5Ma. The stepwise climate spanned 7M-years 25.5Ma through 32.5Ma as depicted in the PaleoTemps chart. The Oligocene climate change was a global increase in ice volume and a 55m decrease in sea level (35.7-33.5Ma) with a closely related (25.5-32.5Ma) temperature depression. The 7M-year depression abruptly terminated within 1-2M-year of the La Garita Caldera volcanism event 28-26 Ma. A deep 400 k-year glaciating Oligocene Miocene boundary event is recorded at McMurdo Sound and King George Island. Paleogeography During this period, the continents continued to drift toward their present positions. Antarctica continued to become more isolated and finally developed a permanent ice cap.(Haines) Mountain building in western North America continued, and the Alps started to rise in Europe as the African plate continued to push north into the Eurasian plate, isolating the remnants of the Tethys Sea. A brief marine incursion marks the early Oligocene in Europe. Oligocene marine exposures are rare in North America. There appears to have been a land bridge in the early Oligocene between North America and Europe since the faunas of the two regions are very similar. During sometime in the Oligocene, South America was finally detached from Antarctica and drifted north towards North America. It also allowed the Antarctic Circumpolar Current to flow, rapidly cooling the continent. Flora Angiosperms continued their expansion throughout the world; tropical and sub-tropical forests were replaced by temperate deciduous woodlands. Open plains and deserts became more common. Grasses expanded from the water-bank habitat in the Eocene and moved out into open tracts; however even at the end of the period it was not quite common enough for modern savanna.(Haines) In North America, subtropical species dominated with cashews and lychee trees were present, and temperate trees such as roses, beech and pine were common. The legumes of the pea and bean family spread, and sedges, bulrushes and ferns continued their ascent. Fauna Important Oligocene land faunas are found on all continents at this time. Even more open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleogene. Marine faunas became fairly modern, as did terrestrial vertebrate faunas in the northern continents. This was probably more as a result of older forms dying out than as a result of more modern forms evolving. Many groups, such as horses, entelodonts, rhinoceroses, oreodonts, and camels, became more cursorial during this time, adapting to the plains that were spreading as the Eocene rainforests receded. South America was isolated from the other continents and evolved a quite distinct fauna during the Oligocene, home to strange animals such as pyrotheres and astrapotheres, as well as litopterns and notoungulates. Sebecosuchian crocodiles, terror birds, and carnivorous marsupials like the borhyaenids remained the dominant predators. Brontotheres died out in the Earliest Oligocene, and creodonts died out outside Africa and the Middle East at the end of the period. Multituberculates, an ancient lineage of primitive mammals, also went extinct in the Oligocene. The Oligocene was home to a wide variety of strange mammals. A good example of this would be in the White River Badlands of the United States, which were formerly a semi-arid prairie home to many different types of endemic mammals, including entelodonts like Archaeotherium, camels (such as Poebrotherium), running rhinos, three-toed horses (such as Mesohippus), nimravids, protoceratids, and early dogs like Hesperocyon. Oreodonts, an endemic American group, were very diverse during this time. In Asia during the Oligocene, a group of running rhinos gave rise to the indricotheres, like Indricotherium, which were the largest land mammals ever to walk the Earth. The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. The fossil record of marine mammals is a little spotty during this time, and not as well known as the Eocene or Miocene, but some fossils have been found. The baleen and toothed cetaceans (whales) just appeared, and their ancestors, the archaeocete cetaceans began to decrease in diversity due to their lack of echolocation, which was very useful as the water became colder and cloudier. Other factors to their decline could include climate changes and competition with today's modern cetaceans and the carcharhinid sharks, which also appeared in this epoch. Early desmostylians, like Behemotops, are known from the Oligocene. Pinnipeds (seals, sea lions, walruses) probably appeared near the end of the epoch from a bear-like or otter-like ancestor. Oceans The Oligocene sees the beginnings of modern ocean circulation, with tectonic shifts causing the opening and closing of ocean gateways. Cooling of the oceans had already commenced by the Eocene/Oligocene boundary, and they continued to cool as the Oligocene progressed. The formation of permanent Antarctic ice sheets during the early Oligocene and possible glacial activity in the Arctic may have influenced this oceanic cooling, though the extent of this influence (or lack thereof) is still a matter of some dispute. Effects of Oceanic Gateways on Circulation The opening and closing of ocean gateways (the Drake Passage, the Tasmanian Gateway, the Tethys seaway, and the Greenland-Iceland-Faroes sill) played a vital part in reshaping oceanic currents during the Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation. Drake Passage The Drake Passage is located between South America and Antarctica. Once the Tasmanian Gateway (between Australia and Antarctica) opened, all that kept the Southern Ocean from being completely isolated was the Drake Passage. The opening of the Drake Passage enabled the formation of the Antarctic Circumpolar Current (ACC), which would have kept cold, Antarctic waters circulating about the continent and strengthened the formation of Antarctic Bottom Water (ABW). With the cold water concentrated around Antarctica, sea surface temperatures and consequently, continental temperatures would have dropped. The onset of Antarctic glaciation (Oi-1) occurred during the early Oligocene, and the effect of the Drake Passage opening on this glaciation has been the subject of much research. However, some controversy still exists as to the exact timing of the passage opening — whether it occurred at the start of the Oligocene or nearer the end. Even so, many theories agree that at the Eocene/Oligocene (E/O) boundary, a yet shallow flow existed between South America and Antarctica, permitting the formation of a preliminary ACC. Stemming from the DOP timing issue is dispute over the extent of the DPO’s influence on global climate. While early researchers concluded that the advent of the ACC was highly important, perhaps even the trigger, for Antarctic glaciation, and subsequent global cooling, other studies have suggested that the δO18 signature is too strong for glaciation to be the main trigger for cooling. Through study of Pacific ocean sediments, other researchers have shown that the transition from warm Eocene ocean temperatures to cool Oligocene ocean temperatures took only 300 ka, which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling Late Oligocene Drake Passage Opening The latest-hypothesized time for the Drake Passage Opening (DPO) is during the early Miocene. Despite the shallow flow between South America and Antarctica, there was not enough of a deep water opening to allow for significant flow to create a true ACC. If the DPO occurred as late as hypothesized, then the ACC could not have had much of an effect on early Oligocene cooling, as it wouldn't have existed. Early-Oligocene Drake Passage Opening The earliest-hypothesized time for the DPO is around 30 Ma. One of the possible issues with this timing was the continental debris, as it were, cluttering up the seaway between the two plates in question. This debris, along with what is known as the Shackleton Fracture Zone, has been shown in a recent study to be fairly young, only about 8 Ma. The aforementioned study concludes that the Drake Passage would be free to allow significant deep water flow by around 31 Ma. This would facilitate an earlier onset of the ACC, but the rapidity of the Currently, an early Oligocene DPO is favored. Tasman Gateway Opening The other major oceanic gateway opening during this time was the Tasman (or Tasmanian, depending on the paper.) Gateway between Australia and Antarctica. The time frame for this opening is less disputed than the Drake Passage and is largely considered to have occurred around 34 Ma. As the gateway widened, the ACC strengthened. Tethys Seaway Closing Though the Tethys was not a gateway, but rather a sea in its own right, its closing during the Oligocene had significant impacts on both ocean circulation and climate. The collision of the African plate with the European plate and of the Indian subcontinent with the Asian plate all conspired to cut off the Tethys seaway that had provided a zonal low-latitude ocean circulation. The closure of Tethys built some new mountains (the Zagros range) and drew down more CO2 from the atmosphere, contributing to global cooling. Greenland-Iceland-Faroes The gradual separation of the clump of continental crust and the deepening of tectonic sill in the North Atlantic that would become Greenland, Iceland, and the Faroe Islands helped to increase the deep water flow in that area. More information about the evolution of North Atlantic Deep Water will be given a few sections down. Ocean Cooling Evidence for ocean-wide cooling during the Oligocene exists mostly in isotopic proxies. Extinction patterns and species migration can also be studied to gain insight into ocean conditions. For a while, it was thought that the Oi-1 event may have significantly contributed to the cooling of the ocean; however, recent evidence tends to deny this. Deep Water Isotopic evidence suggests that during the early Oligocene, the main source of deep water was the North Pacific and the Southern Ocean. As the Greenland-Iceland-Faroe (GIR) sill deepened, connecting the Norwegian-Greenland sea with the Atlantic Ocean, North Atlantic Deep Water began to come into play as well. Model runs suggest that once this occurred, a more modern-looking thermohaline circulation initialized. North Atlantic Deep Water Evidence for the early Oligocene onset of North Atlantic Deep Water (NADW) lies in the beginnings of sediment drift deposition in the North Atlantic, such as the Feni and Southeast Faroe drifts. South Ocean Deep Water South Ocean Deep Water (SODW) began in earnest once the Tasmanian Gateway and the Drake Passage opened fully. Regardless of the time at which the DOP occurred, the effect of cooling the Southern Ocean and leading to increased deep-water formation would have been the same. Impact events Recorded extraterrestrial impacts: *Nunavut, Canada (23 Ma, crater 24 km (15 mi) diameter,) Supervolcanic explosions La Garita Caldera (28 through 26 million years ago, VEI=9.2) Category:Time of Wonder Category:Epoches Category:Oligocene Wonder